The present invention relates to improvements in a dc resistance welder using an inverter.
Conventionally, in inverter type dc resistance welders, the welding current is controlled by a pulse-width modulation (PWM) system. Specifically, the welding current is set to a fixed value depending on the shape of workpieces to be welded and the type of welding guns to be used, and is controlled by an inverter having a fixed frequency determined by a priority condition in which the weight of a welding transformer is minimized on the basis of the fixed welding current. Since the frequency of the inverter is fixed, it is not difficult to design a lightweight welding transformer for performing a welding operation at a fixed welding current. However, in general, a wide variety of workpieces are welded by a single welding machine and thence, various types of welding guns are available to suit the various kinds of workpieces. Thus, there has been a technical limitation in obtaining satisfactory weld quality with a fixed welding current.
For this reason, therefore, when the welding current is varied, new welding transformers have to be designed based on the frequency of the inverter. In such a case, the intended priority condition to minimize the weight of the transformers is no longer maintained, making it difficult to design smaller size transformers.
Further, in the conventional PWM method where the operating frequency of the inverter is fixed, the operating frequency has to be set at a relatively high frequency so as to provide a smaller size welding transformer. As a consequence, the welding current is restrained by the frequency characteristics of the welding transformer and the circuit conditions such as the power cable extending from the power supply to the transformer and the inherent impedance of the transformer. Thus, the welding current can not be passed to the maximum value allowed by design specifications.